It is well known that optical elements for converting natural light from a light source into a unidirectionally-polarized light can be used suitably for liquid crystal projectors.
In a liquid crystal projector, light from a light source illuminates a liquid crystal panel by which the light is modulated with an image and then projected through a projection lens on a screen. Incident light to be modulated in the liquid crystal panel is linearly polarized light which is usually produced through a polarizing plate. In this case, natural light emitted from a natural light source is absorbed by 50% in practice by the polarizing plate disposed at the incident side of the liquid crystal panel, resulting in a considerable loss of brightness of light. The light absorbed in the polarizing plate is converted into thermal energy to increase the temperature of the polarizing plate. To solve the above-mentioned problems, it has been proposed a polarized light converting elemental device that splits natural light into polarized components, matches the components in polarized state and propagating direction and synthesizes again the components to polarized light by using a polarized beam splitter and a cholesteric liquid crystal.
A conventional converting device using a polarized beam splitter is disclosed in Japanese Patent Publication No. 5-58899, or Japanese Utility Model Publication No. 5-26561 or Japanese Laid-Open Patent Publication No. 4-310903, or Japanese Laid-Open Patent Publication No. 5-181135. A conventional converting device using a cholesteric liquid crystal is disclosed in Japanese Laid-Open Patent Publication No. 3-45906.
However, the above-mentioned conventional polarized light converting elements involve the following problems:
In using the polarized light splitter with an optical system disclosed in Japanese Patent Publication No. 5-58899, there take place two different beams propagating in two different directions. Usually, any liquid crystal panel may present an image having a decreased contrast when it works with slant incident light other than light perpendicular to the screen. Consequently, the above-mentioned optical system may decrease the contrast of an image on the liquid crystal panel because of beams other than perpendicular. Furthermore, two separate beams propagate in two different directions and spread wider than the aperture of a projection lens used therein. Consequently, the loss of light occurs to decrease the efficiency of using incident light. In this case, the effective use of two beams can be realized by adopting an enlarged projection lens that is, however, expensive and has a large weight. The optical system has an avoidable enlargement in its size since it is designed to branch light into two separate beams. Consequently, the device with this system may have a large scale in practice.
The polarized-light converting element disclosed in Japanese Laid-Open Patent Publication No. 4-310903 may not cause the enlargement of an optical system but it has complicated construction and is difficult to manufacture.
The prior art devices using a polarized-beam splitter (Japanese Patent Publication No. 5-58899, Japanese Utility Model Publication No. 5-26561 and Japanese Laid-open Patent Publication No. 5-181135) use an expensive polarized-beam splitter and may split light into two linearly polarized beams: one beam having a polarization direction perpendicular to a plane including a light propagating direction and a normal of a beam-splitting surface of the beam splitter and the other having a polarization direction parallel to said plane. Consequently, to match the polarization directions of two beams, it is necessary to change the polarization direction of one polarized-beam by using a half-wave plate (as described in Japanese Patent Publication No. 5-58899 and Japanese Laid-open Patent Publication No. 5-181135) or through three-dimensional reflection by using a mirror or the like means (as described Japanese Utility Model Publication No. 5-26561). However, the effect of a retardation plate considerably depends upon its quality and performance. For example, a retardation plate having uneven characteristics for respective wavelengths may produce light that is differently polarized states depending upon wavelengths. Finally, the converted white-incident light may be colored after passing through polarizing plate. The use of a three-dimensionally disposed mirror may be not effective in practice because of difficulty of synthesizing two converted beams after reflection. Furthermore, this may unavoidably increase the sizes of the system and device.
On the other hand, the conventional polarized light converting elemental device using a cholesteric liquid crystal has the following drawbacks:
The cholesteric liquid crystal has such characteristic that it can selectively reflect either clockwise or anticlockwise circularly polarized component of natural light of a selected range of wavelengths. This characteristic is most noticeably realized with incident light perpendicular to the cholesteric liquid crystal and not completely realized with incident light falling at angles onto the cholesteric liquid crystal.
Consequently, the optical system described in claim 6 of Japanese Laid-open Patent Publication No. 3-45906 can not effectively use the performance of the cholesteric liquid crystal with a beam of light entering the liquid crystal at a large incident angle. In short, the efficiency of using the polarized light is not so much improved. An optical system disclosed in claims 2 to 5 of Japanese Laid-open Patent Publication No. 3-45906, wherein a light beam is split and once fed back to a lamp reflector, can obtain the incident light beam perpendicular to the cholesteric liquid crystal but cannot attain the expected result since the light reflected from the cholesteric liquid crystal is interfered in its optical path by light reflected and dispersed by glass of the lamp and widely spread light from the light source and, in practice, only a small portion of the light can be returned to the cholesteric liquid crystal.
Japanese Laid-Open Patent Publication No. 3-192319 discloses a projection-type display device in which one of two polarized beams separated by the polarized-light separating optical element is doubly reflected to obtain the same polarization direction as that of the other polarized beam. The polarized-light rotating means of the device disclosed in Japanese Laid-Open Patent Publication No. 3-192319 rotates the polarization direction of one linearly polarized beam by doubly reflecting the beam by the mirror whose incident plane (a plane including the optical axis of the incident light and the normal of a mirror surface of the polarized-light rotating means at a point where the optical axis and the mirror surface intersect) is tilted at an angle of 45.degree. from the polarization direction of the linearly polarized beam separated by a polarized-beam splitter. However, the polarization directions of the linearly polarized beams separated by the conventional polarized-light separating means are, respectively, perpendicular and parallel to the paper surface. Consequently, the mirror must be disposed in such a way that the polarization direction of the linearly polarized beam makes an angle of 45.degree. with the incident plane of the mirror. This necessarily causes the optical path of principal ray to be three-dimensionally located. Although the optical path is illustrated as two-dimensionally disposed in Japanese Laid-Open Patent Publication No. 3-192319, two separated beams can not be presented in practice on the same paper surface.